The disclosed invention generally relates to the metallization of pyrolyzable dielectric tapes utilized in the fabrication of multilayer hybrid circuits and structures, and more particularly is directed to a process for filling vias in such pyrolyzable dielectric tapes with metallization without the use of a screen printer.
Hybrid multilayer circuit structures may be made with layered sheets of dielectric tape known in the art as "green tape." Generally, each layer of green tape is individually screen printed to fill preformed vias with metallizaiton and to provide areas of metallization on its surface.
In one known process, referred to as a cofired ceramic process, each layer of green tape is individually punched to form via holes, and then screen printed to provide conductor patterns and to fill via holes with metallization. The individual layers of screen printed green tape are then stacked in the required order, and laminated together using a chosen temperature and pressure. The laminated structure is then fired at a desired elevated temperature.
In another process, identified as the tape transfer process, disclosed in U.S. Pat. No. 4,645,552, issued Feb. 24, 1987, and assigned to the assignee of the present application, a layer of green tape having preformed vias is positioned on and registered to an insulating substrate having, for example, a conductive pattern thereon. The positioned green tape is laminated to the insulating substrate in a laminating press. The laminated structure is then heated to a predetermined elevated temperature sufficient to drive off the organic binder material in the green tape and to securely fuse the inorganic filler material in the green tape to the underlying substrate and the conductive pattern. The conductive pattern and the metallization in the via holes of the laminated green tape are then provided by screen printing.
In both of the foregoing processes, the vias in a layer of green tape are filled with metallization by screen printing. An important consideration with filling vias by screen printing is the importance of precise registration of the print screen with the vias. Changes in relative positions will result in misregistration of the via fill print screen. A further consideration with screen printing via fill is the inability to use very small vias, which precludes denser packing of components. Screen printing via fill may also result in unfilled vias due to a clogged screen.